Projection display systems for light valves

ABSTRACT

Projection display systems for light valves such as liquid crystal display panels, and in particular to the use of color component rotators, such as retardation filters, to provide for improved projection display architectures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 11/044,820 filed Jan. 25, 2005, which is a divisional of U.S.patent application Ser. No. 09/539,918 filed Mar. 31, 2000, to which itclaims priority.

BACKGROUND OF THE INVENTION

The present invention relates to projection display systems for lightvalves such as liquid crystal display panels, and in particular to theuse of color component rotators, such as retardation filters, to providefor improved projection display architectures.

Projection systems for reflective liquid crystal displays (LCDs) aregenerally characterized by their complexity and large size relative tothe systems implemented for transmissive LCDs. FIG. 1A discloses a priorart configuration for a transmissive LCD projector, while FIG. 1B showsa prior art reflective LCD projector for comparison. Dichroic filtersDF1 and DF2 separate the red, green, and blue color components. Thereflective LCDs require a polarizing beamsplitter (PBS) to be placed infront of each LCD in order to reflect light toward the reflective LCD,and then to transmit the modulated light. These components addcomplexity to the system and require the use of a larger distributionand recombination optical system to divide the light into the threecolor channels (i.e. the optical paths traveled by the three colorcomponents such as red, green, and blue).

An alternative system for reflective LCDs divides the illumination intothe three color channels and recombines the output distributions into asmaller and less complex system. The basic configuration is shown inFIG. 2. However, the illumination input to this system must have a veryspecific distribution of color components and polarizations in which twoof the color components (green and blue) are polarized in one direction,and the other color component (red) is polarized orthogonally to theother two. In order to produce that combination of color components andpolarizations, a complicated prefiltering system is needed.

One such system is shown in FIG. 2, and indicates that considerablecomplexity is added back to the system in order to implement theprefiltering. In the system shown in FIG. 2, only one-half of the lightis used, since the unwanted polarization state of each color componentis simply discarded. In order to increase brightness of the system, amore complex prefiltering system is required that recycles the polarizedlight.

Various architectures have been proposed for projection display systems.Ledebuhr, U.S. Pat. No. 4,687,301, and Ledebuhr, U.S. Pat. No.4,836,649, both describe projection systems for liquid crystal lightvalve (LCLV). The LCLV is an optically addressed reflective LC modulatorand the systems described in these patents show optics to split a lightsource into separate colors paths and then individually illuminate andproject the three LCLV devices. Both of these systems use only one-halfof the illumination light since the unwanted polarization state isinitially discarded. Ledebuhr, U.S. Pat. No. 4,687,301, uses acomplicated color separating system to direct the color components tothe LCLVs. Ledebuhr, U.S. Pat. No. 4,836,649 uses simpler but morenumerous elements resulting in a large projection system.

Doany, et al. U.S. Pat. No. 5,621,486, and Dove, U.S. Pat. No.5,658,060, both describe architectures that use Philips type prisms tocontrol the three separate color channels. Doany, et al. U.S. Pat. No.5,621,486, uses a single PBS prism to control the light into and out ofall three LCIs) and a Philips prism to both split up and recombine thecolor channels. This arrangement appears simple, but the control ofcolor in a Philips prism for p-polarization on the input ands-polarization on the output is extremely difficult, and no successfulimplementation of this type of system exists. Dove, U.S. Pat. No.5,658,060, places a P13S prism in front of each LCD and uses the Philipsprism only to recombine the color channels. This requires a secondoptical arrangement to split up the color distributions and leads to alarger, more complicated system overall.

Ooi et al., U.S. Pat. No. 5,648,860, uses an offset illumination andprojection scheme. The system does not use a PBS prism, but insteadrelies on the offset to separate the input and output lightdistributions. The color splitting and recombination is accomplished bytilted dichroics that perform essentially the same as the dichroics in aPhilips prism, with the same polarization related problems.

Hattori et al., U.S. Pat. No. 5,798,819, and Ueda, U.S. Pat. No.5,918,961, both describe minor variations of the typical reflective LCDprojector of FIG. 1B. These systems use a crossed dichroic prism torecombine light from the three LCDs and a separate crossed dichroicarrangement to perform the color splitting from the illumination system.

Sharp, U.S. Pat. No. 5,751,384, describes techniques for makingwaveband-specific retardation filters. This patent also describes asingle panel LCD projector using the retardation filters in an activecolor shutter to gate the three colors onto the LCD for color fieldsequential projection.

Nevertheless, there remains a need for a bright projection displaysystem, preferably for reflective LCD panels that utilize a smallarchitecture. What is therefore desired is a projection display that isas small as or smaller than conventional projection displays, is capableof utilizing reflective LCD panels, uses readily available opticalelements that perform well, uses conventional polarization converters,and provides good contrast without sacrificing brightness.

BRIEF SUMMARY OF THE INVENTION

The present invention overcomes the drawbacks of the prior art byproviding in a first aspect of the invention a projection display systemhaving a light source, a polarization converter, at least one polarizingbeamsplitter, at least one liquid crystal display panel for generatingan image, a projection source for projecting the image, and a colorcomponent rotator located between the polarization converter and theprojection source.

In a second separate aspect of the invention, a projection displaysystem has a light source, a polarization converter, at least twopolarizing beamsplitters, at least three liquid crystal display panels,each for generating a respective image, a projection source forprojecting the images, and at least two color component rotators, eachof the color component rotators located between the polarizationconverter and the projection source.

In a third separate aspect of the invention, a method of displaying animage is provided. First, light comprised of at least a first, second,and third color component is provided. The light is converted topolarized light having a single polarization state. The first colorcomponent is separated from the second and third color components. Thepolarization state of the second color component is changed relative tothe third color component. The second color component is separated fromthe third color component. Respective images are generated from each ofthe three color components. The images are then combined and projected.

The various aspects of the present invention each have one or more ofthe following advantages. The systems achieve their advantages throughthe use of color component rotators, or wavelength-specific retardationfilters, located within the optical systems to control the polarizationorientation of one of the color components in the system relative to theother two. The use of a color component rotator allows the polarizationorientation of the three color components to be controlled within themain color distribution and recombination portion of the system ratherthan in a prefiltering system included within the illumination optics.This allows the use of conventional polarization converters rather thancomplicated prefiltering systems.

In addition, the use of color component rotators enables smallerdistribution and recombination systems than conventional reflectiveprojection display systems and, in one case, smaller even than typicaltransmissive projection display systems. Thus, the projection displaysystems and methods of the present invention reduce the overallprojection display system size and complexity. The systems provide theseadvantages while achieving good contrast and without sacrificingbrightness.

The foregoing and other objectives, features, and advantages of theinvention will be more readily understood upon consideration of thefollowing detailed description of the invention, taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a schematic diagram of a prior art transmissive LCDprojector.

FIG. 1B is a schematic diagram of a prior art reflective LCD projector.

FIG. 2 is a schematic diagram of a projection display system thatutilizes a prefiltering system.

FIG. 3 is a schematic diagram of one embodiment of a projection displaysystem of the present invention.

FIG. 4A is a second embodiment of a projection display system of thepresent invention.

FIG. 4B is a detail view of the distribution and recombination portionof the display of FIG. 4A.

FIG. 5 is a third embodiment of a projection display system of thepresent invention.

FIG. 6 is a fourth embodiment of a projection display system of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to the figures, wherein like numerals refer to likeelements, FIG. 3 shows an exemplary projection display 10 having anillumination portion 11 and distribution and recombination portion 22.Distribution and recombination portion 22 includes three reflectiveliquid crystal display (LCD) panels 14, 16, and 18, also referred to asliquid crystal light valves.

Illumination portion 11 includes a light source 12 for producing whitelight, which may be separated into different color components ofdifferent bandwidths, such as a red color component, blue colorcomponent, and green color component. The white light from light source12 passes through a polarization converter shown generally at 20.Polarization converter 20 may take the form of any conventionalpolarization converter, so that the randomly polarized light from lightsource 12 is converted into a single polarization state. In theembodiment shown in FIG. 3, the polarization converter 20 converts thewhite light from randomly polarized light into light that is polarizedin the S direction. Polarization converter 20 is a conventionalpolarization converter structure comprised of fly's-eye lens plates 24,26 and polarization converter prism array 28. Other polarizationconverters may also be used. While not preferred, the present inventioncould be used with a polarizing filter to produce uniformly polarizedlight; however, only half the light from light source 12 would be used.The polarized light is directed from the polarization converter 20toward a mirror 30, which reflects the light through a lens 32.

The now S-polarized light exits the illumination portion 11 of thedisplay 10 and enters the distribution and recombination portion 22. Thewhite light encounters dichroic filter 34, which separates the red colorcomponent from the blue and green color components. In general, dichroicfilters transmit light of a certain bandwidth, and reflect light ofanother band width. In display 10, dichroic filter 34 transmits the redcolor component while reflecting the blue and green color components.

Referring now to the blue and green channels, after reflecting fromdichroic filter 34 the blue and green light components pass through afield lens 36, which in combination with other elements controls thesize of the light projected on the LCD panel. The blue and green colorcomponents then pass through a polarizer 38 that transmits onlyS-polarized light. The polarizer 38 improves contrast by filtering outP-polarized light that otherwise may leak through to the LCD panels 16and 18.

The blue and green color components then pass through a selective colorcomponent rotator 40, which rotates one of the color components (e.g.,the blue color component) from one polarization state (e.g., theS-polarization state) to another polarization state (e.g., theP-polarization state). The color component rotator is a wavebandspecific retardation filter. It is a specially designed stack ofretardation films in which the amount of retardation imparted todifferent wavebands can be selectively controlled by the orientation andnumber of retardation films used. The details of the design andoperation of such color component rotators are described in Sharp, U.S.Pat. No. 5,751,384. A retardation filter can be made to act like a halfwaveplate for one bandwidth of light while leaving light of all othercolors or bandwidths unaffected. Such color component rotators may beobtained from Color Link in Boulder, Colo., or Cambridge Research &Instrumentation in Cambridge, Massachusetts. In the projection display10, color component rotator 40 acts as a half waveplate for the bluecolor component, thus rotating the blue color component 90°, but leavingthe green color component unaffected. Accordingly, the color componentrotator 40 rotates the blue color component from the S to P-polarizationstate, while the green color component remains in the S-polarizationstate.

The blue and green color components then enter a polarizing beamsplitter41 having different polarization states (e.g., S and P, respectively).The polarizing beamsplitter reflects the S-polarized green colorcomponent while transmitting the P-polarized blue color component. Thegreen color component reflecting off the polarizing beamsplitter 41 isimaged using green LCD panel 16. The green image reflected by themodulated LCD panel 16 is in the P-polarization state and is transmittedthrough polarizing beamsplitter 41 and into the crossed dichroic prism42.

Returning to the blue channel, the blue color component transmittedthrough the polarizing beamsplitter 41 is transmitted through relay lens44, reflected off mirror 46 and transmitted through a second relay lens48. The blue color component passes through another selective colorcomponent rotator 50, which rotates the polarization of the blue colorcomponent back to the S-polarization state. The now S-polarized bluecolor component then passes through a polarizer 51 which transmitsS-polarized light. The S-polarized blue color component then enters athird polarizing beamsplitter 52, which reflects the S-polarized bluecolor component onto the blue LCD panel 18. The blue image reflected byblue LCD panel 18 is in the P-polarization state and is transmittedthrough polarizing beamsplitter 52 into the crossed dichroic prism 42.The relay lenses 44 and 48 are used to compensate for the longer pathlength of the blue channel relative to the green and red channels.

Returning to the red channel, the red color component is transmitted bydichroic filter 34 and is focused by a field lens 54. The red colorcomponent then passes through polarizer 56, which is oriented totransmit S-polarized light. The polarizer 56 improves contrast byeliminating P-polarized light that might otherwise leak through to LCDpanel 14. The red color component then enters polarizing beamsplitter 58which reflects the S-polarized light into the red LCD panel 14. Themodulated LCD panel 14 generates a red image. The reflected red image(in the P-polarization state) passes through the beamsplitter 58 andinto the crossed dichroic prism 42.

The three color components reflected from the three LCD panels 14, 16,and 18 pass through their respective polarizing beamsplitters and intothe crossed dichroic prism 42, which combines the reflected images. Theprojection lens 62 then projects the converged images from all three LCDpanels onto a projection screen (not shown).

The projection system of FIG. 3 retains the configuration of polarizingbeamsplitters and crossed dichroic prism of the conventional reflectiveLCD projector shown in FIG. 1B. However, this projector significantlyreduces the size of the optics required to distribute the illuminationlight into the three color channels. The key to this reduction is theuse of the color component rotator 40, in this case a blue colorcomponent rotator, which allows the single polarizing beamsplitter 41 toperform the dual function of separating the green and blue colorcomponents and also to control the operation of green LCD panel 16.Thus, the system 10 has significantly reduced the size and complexity ofthe optics required.

An alternative projection display system 10A is shown in FIGS. 4A and4B. Like system 10 shown in FIG. 3, system 10A shown in FIG. 4A has alight source 12, red LCD panel 14, green LCD panel 16, and blue LCDpanel 18. The system 10A utilizes the same illumination portion 11. Apolarization converter 20 comprised of fly's-eye lens plates 24, 26 andprism array 28 provide light consisting of all three color componentspolarized in the S direction. Mirror 30 reflects light through lens 32into the distribution and recombination portion 22A of the system.

Referring now to the distribution and recombination portion 22A shown inmore detail in FIG. 4B, dichroic filter 134 separates the white light bytransmitting the green color component while reflecting the red and bluecolor components. Referring to the green channel, the green colorcomponent passes through polarizer 156 which transmits light polarizedin the S direction. The green color component then enters polarizingbeamsplitter 158 which reflects the S-polarized light onto the green LCDpanel 16. The reflected image (now in the P-polarization state) passesthrough the beamsplitter 158 and through analyzer 160, which transmitslight in the P-polarization state. Analyzer 160 improves contrast byeliminating S-polarized light that has leaked through the green channel.The green color component is then reflected by dichroic filter 164, andthen transmitted through projection lens 62.

Turning to the red and blue channels, the red and blue color componentsare reflected by dichroic filter 134 and passed through polarizer 136,which transmits only S-polarized light. The red and blue colorcomponents then pass through a selective color component rotator 138,that acts as a half waveplate for the blue color component. Thus, theblue color component is rotated from the S-polarization state to theP-polarization state, while the red color component remains unaffected.Thus, the two color components (red and blue) entering the polarizingbeamsplitter 140 have different polarization states (e.g. S and Prespectively). The polarizing beamsplitter 140 reflects the red colorcomponent and transmits the blue color component. The polarizingbeamsplitter 140 reflects the S-polarized red color component onto themodulated red LCD panel 14, which generates a red image now in theP-polarization state. Similarly, the blue color component transmitted bythe polarizing beamsplitter 140 is reflected off the modulated blue LCDpanel 18, which generates a blue image in the S-polarization state. Thered image reflected by LCD panel 14 is transmitted through polarizingbeamsplitter 140 while the blue image reflected by LCD panel 18 isreflected by the polarizing beamsplitter 140. Both the red and bluecolor components pass through another selective color component rotator150, which selectively rotates the blue color component from the S tothe P-polarization state, so that the two color components again havethe same polarization state. The red and blue color components then passthrough an analyzer 152 which transmits only light that is P-polarized.The blue and red color components then pass through dichroic filter 164,where they are combined with the green color component and projectedthrough projection lens 62.

The analyzer 152 and the second selective color component rotator 150are introduced to control a practical implementation problem that arisesdue to the non-ideal operation of the polarizing beamsplitter 140.Ideally, a polarizing beamsplitter will reflect all S-polarized lightthat enters and transmit all P-polarized light. However, a typicalpractical polarizing beamsplitter has extremely high reflectivity forS-polarized light with virtually no S-polarized light transmitted. Thetransmitted light is therefore a highly pure, P-polarized distribution.However, the practical polarizing beamsplitter also reflects a smallportion of the P-polarized light, sometimes as much as 10 percent,giving a reflected distribution that is a mixture of predominantlyS-polarized light and a small portion of P-polarized light.

Turning now to the projection display system shown in FIG. 4B, theselective color component rotator 138 rotates the blue color componentfrom the S to P-polarization state, leaving the red color componentS-polarized. The light transmitted through polarizing beamsplitter 140will be just the blue color component with the P-polarization, but thereflected light will be the S-polarized red color component and a smallportion of the P-polarized blue color component. Accordingly, a portionof the blue color component leaks into the red channel and illuminatesthe red LCD panel 14. If the blue color component reflects off the redLCD panel 14 without modulation, it will re-enter polarizingbeamsplitter 140 as P-polarized light and transmit through thepolarizing beamsplitter 140, through the dichroic filter 164, andthrough the projection lens 62. This undesired light will significantlyreduce the contrast in the blue color component.

However, by introducing the second selective color component rotator150, also designed like the color component rotator 138 to be a halfwaveplate for the blue color component, the unwanted P-polarized bluecolor component from the red channel will be rotated to be S-polarized.The desired output blue color component from the blue channel willreflect out of the polarizing beamsplitter 140, as S-polarized and willbe rotated to the P-polarization state by the selective color componentrotator 150. The blue output distribution then has the same P-polarizedorientation as the desired red output distribution. The analyzer 152,oriented in the P direction, transmits the desired red and blue colorcomponents in the P-polarization state, but absorbs and eliminates theunwanted blue color component in the P-polarization state that leakedinto the red channel and reflected off the red LCD panel 14 throughpolarizing beamsplitter 140. Some portion of the blue color componentthat leaks into the red channel may be modulated by the red LCD panel 14and re-enter the polarizing beamsplitter 140 as S-polarized light. Ifthis occurs, the polarizing beamsplitter 140 has strong S-polarizedreflection and virtually no S-polarized transmittance. The modulatedportion of the blue leakage color component will then be reflected backtoward the illumination optics and will not pass through to theprojection lens. This particular configuration of using two selectivecolor component rotators 138, 150 and an analyzer 152 is essential toenable the high contrast operation of two LCD panels with a single PBSprism.

Another alternative projection display system is shown in FIG. 5. Thesystem again begins with a conventional illumination system as describedpreviously for the embodiments of FIGS. 3 and 4A and 4B. The input colorcomponents, all S-polarized, enter the color distribution andrecombination portion 22B of the system 10B. A green transmittingdichroic filter 134 reflects the blue and red color components up to thepolarizing beamsplitter 140. The polarizer 136 and analyzer 152, theselective color component rotators 138 and 150, and the polarizingbeamsplitter 140 control the operation of splitting and recombining thered and blue color components in exactly the same fashion as in thesystem 10A of FIGS. 4A and 4B.

The alternative arrangement is contained within the green channel. TheS-polarized green color component is passed through lens 200 and isreflected by mirror 202. The green color component then is passedthrough lens 204 and polarizer 206. The polarizer 206 removes anyresidual P-polarized light. The green color component then passesthrough a selective color component rotator 208, which is designed andoriented to rotate green light polarization by 90 degrees, so as torotate the green color component to the P-polarization state. The greencolor component passes through polarizing beamsplitter 216 to the greenLCD panel 16. The relay lenses 200 and 204 are used to compensate forthe longer path length of the green channel relative to the red or bluechannels. A block of glass 212 is introduced to provide the same opticalpath length for the green channel as the red and blue channels betweenthe LCD panels and the projection lens. The modulated LCD panel 16generates a green image, which is reflected in the S-polarization state.The green image reflects off the polarizing beamsplitter 216 and into acolor component rotator 210. This color component rotator 210selectively rotates the polarization of the green color component fromthe S to the P state. The analyzer 214 eliminates any green light thatmight have been reflected into the red or blue channel from polarizingbeamsplitter 216 and maintains high contrast performance for the greenchannel.

While exemplary projection displays have been described, otherprojection display configurations that utilize LCD panels (eitherreflective, transmissive, or a combination thereof) and polarizingdevices such as polarization converters may find utility with thepresent invention. Moreover, other color components, wavelength ranges,and polarization states may be used as desired.

Other alternative system architectures are also possible. In the systemshown in FIG. 4B, dichroic filter 134 may instead transmit the red orblue color component. Alternatively, the output dichroic filter 164could be changed to a green transmitting filter 164A, rather than agreen reflecting filter. The resulting system configuration is shown inFIG. 6. The projection lens 62 is moved to capture the output and sendthe projected image up, rather than to the right, and may be consideredfor overall system packaging considerations.

With respect to the embodiment shown in FIG. 3, it may be possible toswitch the red reflecting dichroic in the crossed dichroic prism 42 to agreen reflecting dichroic. Then, by changing the input dichroic filter34 to a green transmitting filter, the LCD panels 14 and 16 may besubstituted for each other, so that the green color component wouldenter from the bottom as shown in FIG. 3 while the red color componentwould enter the crossed dichroic prism 42 from the left as shown in FIG.3.

The terms and expressions which have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention, in the use of such terms andexpressions, of excluding equivalents of the features shown anddescribed or portions thereof, it being recognized that the scope of theinvention is defined and limited only by the claims which follow.

1. A method for displaying an image, comprising: (a) providing lightcomprised of a first color component, a second color component, and athird color component; (b) converting said light to a singlepolarization state; (c) separating said first color component from saidsecond and third color components while said first, second, and thirdcolor components are in the same beam; (d) mismatching said polarizationstates of said second and third color components relative to each otherwhile said second and third color components are within the same beam;(e) separating said second color component from said third colorcomponent while said second and third color components are within thesame beam; (f) generating respective images from each of said first,second, and third color components separated from one another intodifferent beams; and (g) projecting said images.
 2. The method of claim1, wherein said first, second and third color components are green,blue, and red respectively.
 3. The method of claim 1, wherein said firstcolor component is separated from said second and third color componentusing a dichroic filter.
 4. The method of claim 3, wherein said secondcolor component is separated from said third color component using apolarizing beamsplitter.
 5. The method of claim 1, wherein saidpolarization state of said second color component is changed using acolor component rotator.
 6. The method of claim 1, wherein said first,second and third color components are reflected onto respective liquidcrystal display panels to generate said images.
 7. The method of claim6, wherein said first, second and third color components are reflectedonto respective liquid crystal display panels using only two polarizingbeamsplitters.
 8. The method of claim 1, further comprising the step ofchanging the polarization state of said first color component beforegenerating said image from said first color component.
 9. The method ofclaim 8, further comprising the step of changing the polarization stateof said first color component again after generating said image fromsaid first color component.